Many common construction and utility operations that require soil excavation depend on the operator's knowledge of the orientation and depth of underground utility lines, pipelines and cable lines. Because contact by excavating equipment is almost invariably damaging to underground lines, it is very important to know the exact position of those lines prior to commencing digging activities. Knowledge of underground line position allows the operator to avoid coming in contact with and damaging such lines.
There are several known locating systems that are currently used to locate underground lines. Most of the known locators involve receiver detection of a magnetic field derived from electrical current directly fed or induced onto an underground line.
The magnetic field lines emanating from a line are essentially cylindrical in shape with the center of the cylinder being the current-carrying line itself. As the current flows along the line, losses occur as a result of displacement and induction of currents into the soil. When the rate of loss along the line length is not great, depth can be computed through the use of a signal strength ratio. For lines that run straight underground along a certain depth, the magnetic field strength is inversely proportional to the distance from the line to the receiver. Depth is typically determined by taking two signal strength readings at different locations directly above the line.
U.S. Pat. No. 6,756,784 to Mercer et al. describes a locator/monitor that is capable of locating a boring tool and monitoring the progress of the tool for control purposes. The locator/monitor described in Mercer achieves its goals through the operation of an antenna assembly that features one cluster of two orthogonal antennas that are in spatial proximity to each other and not a fixed distance apart.
The locator/monitor described in Mercer operates in the following way: (1) an operator locates the underground transmitter (mounted on a boring tool); (2) operator deploys the receiver/locator at a first height above the transmitter location and measures the magnetic field strength emanating from the transmitter; and (3) deploys the receiver at a second height to measure the magnetic field strength emanating from the transmitter. Although this device may be accurate, its use is time-consuming, as the operator has to take measurements at two different heights.
U.S. Pat. No. 6,768,307 to Brune et al. describes a system for flux plane locating that includes a boring tool with a transmitter that transmits a locating field such that the locating field exhibits a pair of locate points in relation to the surface to the ground and a portable locator that is used to measuring the intensity of the locating field. The locator contains one antenna cluster that contains three orthogonally disposed antennas, two in a horizontal plane, and one disposed in a vertical plane. As with the previous device, the use of this locator suffers from the disadvantage of being time consuming, as it forces the operator to move the locator to several positions before arriving at the final determination of line location or depth.
There are also presently known line-locating devices that employ two antennas and logic circuitry to determine depth. The antennas are separated by a fixed distance. Because the separation distance is known, cable depth can be computed by interpreting the magnetic field strength.
U.S. Pat. No. 5,920,194 to Lewis et al. describes a locator that has spaced antennas that detect electromagnetic signals from an underground line. The locator contains a processor that analyzes the electromagnetic signals and determines the separation of the locator and the underground line, both in terms of the direction corresponding to the spacing of the antennas and the perpendicular direction to the underground line. The display on the device shows the separation of the locator and the object.
This device suffers from several disadvantages. First, because the device determines the separation between itself and the underground line by measuring the angles between its antennas and the surface of the underground solenoid, the angle of the positioning of the locator becomes critical. Lewis et al. attempts to solve that problem by including a tilt sensor in its device, which in all likelihood makes the device more expensive.
Also, the locator is preferably a ground penetration probe that is driven into the ground to maintain a constant angular position during measurements. This may be impractical in certain locations where the ground is difficult to penetrate. A further disadvantage of this locator is that it provides a relatively precise measurement only after requiring the operator to measure the separation of the locator and the underground line at two different locator positions and comparing the two results. Thus, the operation of the device in Lewis is time consuming and prone to unnecessary errors if the locator is not identically positioned at both measurement locations.
Measurement accuracy in devices of the prior art is often affected by differential drifting of the electronics associated with the antennas as well as by differential responses of the antennas themselves. To increase sensitivity, ferrite rods are sometimes employed to enhance the effective capture area of the antennas. As a result of the antenna separation, both antennas may not experience the same thermal environment. This can be a problem because the characteristics of ferrite vary measurably with temperature and are not consistent between rods.
There are other problems to be overcome when measuring AC flux fields using multiple ferrite antennas. For example, synchronization is a major problem. Because AC signals are characterized by amplitude and phase, both must be accounted for to get accurate results. If the phases of signals being measured on separate antennas are somehow shifted relative to each other without account, then there is no way of knowing at any given instance what the actual relative responses of those antennas are. Often, amplifying and filtering circuits will by their very nature introduce phase shifts that are not controllable enough to maintain accurate resulting signals on the back end. A second problem of using ferrite antennas is measurement accuracy and calibration. This is because of variations in ferrite permeability, winding anomalies, etc.
Therefore, a need exists for a digital locating system that can not only accurately predict the location, orientation and depth of an underground line using simple and quick procedure, but one that can also adequately account for anomalies in the ferrite antennas and component tolerance errors in the processing circuitry itself.